Method for measuring recirculated exhaust gas flow in a compression-ignition engine

ABSTRACT

A system and method for measuring the flow rate of recirculated exhaust gas in a compression ignition engine including a plurality of engine sensors having outputs indicative of current engine conditions and a turbocharger. The system includes an exhaust gas recirculation (EGR) valve mounted in the exhaust pipe upstream of the turbocharger for diverting a selectable portion of the exhaust gas for recirculation and combination with the charge air, one or more sensors for sensing current conditions of the recirculated exhaust gas, including temperature and pressure, one or more sensors for sensing current conditions of the intake air, and control logic for determining the flow rate of the recirculated exhaust gas as a function of the sensed conditions. In one embodiment, the system includes an obstruction in the flow path of the recirculated exhaust gas and a differential pressure sensor for determining the pressure differential between a point upstream of the obstruction and a point downstream of the obstruction and control logic for determining the flow rate of the recirculated exhaust gas as a function of the current intake manifold pressure, the recirculated exhaust gas temperature, and the differential pressure drop across the obstruction.

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application is a continuation of U.S. application Ser. No.09/641,256, filed Aug. 17, 2000 which claims the benefit of U.S.provisional application Serial No. 60/193,837 filed Mar. 31, 2000.

TECHNICAL FIELD

[0002] The present invention relates to systems and methods forcontrolling the ratio of the mixture of recirculated exhaust gas andintake air in a compression-ignition engine utilizing a turbocharger,and, in particular, a system and method for determining the flow rate ofthe recirculated exhaust gas.

BACKGROUND ART

[0003] In compression-ignition engines, such as heavy-duty dieselengines, the intake air is typically cooled and compressed, typically byusing a turbocharger, to provide increased power density for the engine.Added flexibility in the compression of the intake air over aconventional turbocharger is often achieved by using a variable geometryturbocharger which may be controlled by the engine's electronic controlmodule (“ECM”) to supply varying amounts of turbo boost pressure to theengine, depending on various operating conditions. One system forcontrolling an engine having a VGT is disclosed in U.S. Pat. No.6,000,221, issued to Church et al. on Dec. 14, 1999.

[0004] One important objective for compression-ignition engine designersis to reduce NO_(X) emissions, while minimizing the negative impact onengine fuel economy and durability.

DISCLOSURE OF INVENTION

[0005] It is therefore an object of the present invention to provide asystem and method for reducing NO_(X) emissions in acompression-ignition engine employing a turbocharger.

[0006] It is another object of the present invention to provide a systemand method for measuring the flow rate of exhaust gas which isrecirculated for combination with intake air in a compression-ignitionengine.

[0007] In carrying out the above objects and other objects and featuresof the present invention, a system and method are provided for measuringthe flow rate of recirculated exhaust gas in a compression-ignitionengine including a plurality of engine sensors having outputs indicativeof current engine conditions and a turbocharger. The system includes anexhaust gas recirculation (EGR) valve mounted in the exhaust pipeupstream of the turbocharger for diverting a selectable portion of theexhaust gas for recirculation and combination with the charge air, oneor more sensors for sensing current conditions of the recirculatedexhaust gas, including temperature and pressure, one or more sensors forsensing current conditions of the intake air, and control logic fordetermining the flow rate of the recirculated exhaust gas as a functionof the sensed conditions.

[0008] In one embodiment, the system includes an obstruction in the flowpath of the recirculated exhaust gas, a temperature sensor mounted forsensing the temperature of the recirculated exhaust gas, and adifferential pressure sensor including a first pressure tap located forsensing the pressure of the recirculated exhaust gas upstream of theobstruction and a second pressure tap located for sensing the pressureof the recirculated exhaust gas downstream of the obstruction, andwherein the control logic includes logic for determining the flow rateof the exhaust gas as a function of the differential pressure dropacross the obstruction and the exhaust gas temperature.

[0009] In another embodiment, the system includes a first temperaturesensor mounted for sensing the temperature of the recirculated exhaustgas, a second temperature sensor mounted for sensing the temperature ofthe charge air, and a third temperature sensor mounted for sensing thetemperature of the mixture of charged air and recirculated exhaust gas,and wherein the control logic includes logic for determining the flowrate of the recirculated exhaust gas as a function of the temperaturessensed by the first sensor, the second sensor, and the third sensor.

[0010] One embodiment of the system employs a thin plate obstructorwhich defines a relatively small diameter orifice in the exhaust gasrecirculation pipe, thereby creating a relatively high pressure drop asthe gas flows through the orifice. The thickness of the obstructorpreferably ranges from about to 0.03 to about 0.08 pipe diameters, andmost preferably is about 0.05 pipe diameters. The orifice defined by theobstructor is a circular opening having a diameter between about 60 and80 percent of the pipe diameter, and most preferably about 60 percent ofthe pipe diameter. In one embodiment, the edge of the plate defining theorifice is beveled to achieve a sharper edge on the orifice plate, tothereby reduce diesel particulate deposits on the edge defining thecritical diameter of the obstructor. It will be appreciated that theembodiment of the present invention which utilizes this thin-wall,sharp-edged obstructor offers relatively greater accuracy over thesensor life, since the thin wall and sharp edge design of the obstructorminimizes the amount of diesel particulate deposits on the edges of theorifice obstructor which define the orifice. Such deposits would, overtime, reduce the effective orifice area and, thereby, reduce thesystem's accuracy since, as is shown hereinafter, the EGR flow ratedetermination utilizes constants which are calibrated for the specificgeometry.

[0011] In the differential pressure embodiment, the flow rate of therecirculated exhaust gas is determined from the voltage input from thedifferential pressure sensor, and from the sensed recirculated exhaustgas temperature, according to the following relation:

EGR Flow Rate (kg/min)=(EGR Gas Density/DensityCorrection)^(a)*b*(Differential Pressure Drop, kPa)^(c)

[0012] where the Density Correction, a, b and c are each calibratableconstants for a particular orifice design.

[0013] One advantage of employing the embodiment of the presentinvention that determines the flow rate as a function of thedifferential pressure drop across an obstruction is that the currentlyavailable pressure sensors provide relatively more accurate readings ina relatively faster response time than other sensors. Thus, the EGR canbe reliably determined even during transient engine operationconditions.

[0014] The embodiment of the present invention which employs sensedtemperature differential utilizes sensor inputs from each of the chargeair temperature sensor, recirculated exhaust gas temperature sensor, andcharge air/recirculated exhaust mixture temperature sensor, determinesthe recirculated exhaust gas flow ratio (EGR %) according to thefollowing relationship:

[0015] The recirculated exhaust gas flow ratio${{EGR}\quad \%} = {\frac{{\overset{.}{m}}_{egr}}{{\overset{.}{m}}_{air}} = \frac{T_{mixture} - T_{air}}{T_{egr} - T_{mixture}}}$

[0016] It will be appreciated that one advantage of employing thisembodiment of the present invention is that the system is non-intrusiveto exhaust gas recirculation flow and results in a nearly non-existentpressure drop in the system.

[0017] In another embodiment of the present invention employs adifferential pressure system of the type described above in conjunctionwith a differential temperature system so that, during steady stateoperation of the engine, the differential temperature system can be usedto cross calibrate the differential pressure system.

[0018] The measuring system may be integrated with an engine controlmodule (ECM) to provide an accurate EGR flow measurement as input to theECM which can be used as feedback for the EGR valve, and/or the VGTcontroller to adjust the EGR valve and/or VGT vane positions and,consequently, control the rate of exhaust gas recirculation in a closedloop.

[0019] It will thus be appreciated that the system of the presentinvention allows for an accurate EGR flow measurement, thereby providingclosed-loop controller feedback and input by which suitable controllogic can detect a malfunction or tampering with the EGR flow circuit.

[0020] The above objects and other objects, features and advantages ofthe present invention are readily apparent from the following detaileddescription of the best mode for carrying out the invention when takenin connection with the accompanying drawings.

BRIEF DESCRIPTION OF DRAWINGS

[0021]FIG. 1 is a schematic diagram of the system of the presentinvention;

[0022]FIG. 2 is a more detailed schematic illustrating potentiallocations of the sensors and obstructor in the recirculated exhaust gasand the charge air intake of the engine;

[0023]FIG. 3 is a schematic diagram of the obstructor employed in oneembodiment of the present invention;

[0024]FIG. 4 is a cross sectional view, at 4-4, of the exhaust gasrecirculation pipe and obstructor shown in FIG. 3;

[0025]FIG. 5 is a block diagram illustrating the control logic fordetermining the EGR flow rate utilizing the differential pressure dropmethod of the present invention;

[0026]FIG. 6 is a partial cross-sectional view of two embodiments of theedge of the obstructor shown in FIGS. 3 and 4;

[0027]FIG. 7 is a schematic diagram illustrating one embodiment of thepresent invention which measures the EGR flow ratio as a function ofcharge air temperature, recirculated exhaust gas temperature, and chargeair/exhaust gas mixture temperature; and

[0028]FIG. 8 is a block diagram illustrating the control logic for thedifferential temperature embodiment of the present invention.

BEST MODE FOR CARRYING OUT THE INVENTION

[0029] Referring now to FIG. 1, a system for controlling a compressionignition engine including the present invention is shown. The system,generally indicated by reference numeral 10, includes a compressionignition engine 12 having a plurality of cylinders, each fed by a fuelinjector 14. The engine typically utilizes 4, 6, 8, 12, 16, or 24cylinders, but may employ any other number of cylinders as desired. Thefuel injectors 14 receive pressurized fuel from a supply connected toone or more high or low pressure pumps (not shown) as is well known inthe art. Alternatively, the system may employ a plurality of unit pumps(not shown), each pump supplying a fuel to one of the injectors 14.

[0030] The system 10 includes a variable geometry turbocharger 50 fordrawing air into the cylinders to create increased power duringcombustion. Engine exhaust gas is routed to the turbocharger turbineinlets along line 56. Air drawn into the engine air intake is routedthrough the compressor and to the engine through air inlet lines 58. Itshould be appreciated that the present invention may be employed with anengine which utilizes any conventional turbocharger. However, in thepreferred embodiment the system is employed with a diesel engine and avariable geometry turbocharger (“VGR”). It also is to be understood thatthe single turbocharging system is shown for purposes of illustration,and that systems and methods of the present invention may be employed ina multiple turbocharging system.

[0031] The system 10 also includes various sensors 20 for generatingsignals indicative of corresponding operational conditions or parametersof the engine 12, the vehicle transmission (not shown), turbocharger 50,and/or other vehicular components. The sensors 20 are in electricalcommunication with a controller 22 via input ports 24. Controller 22preferably includes a microprocessor 26 in communication with variouscomputer readable storage media 28 via data and control bus 30. Computerreadable storage media 28 may include any of a number of known deviceswhich function as a read only memory (ROM) 32, random access memory(RAM) 34, keep-alive memory (KAM) 36, and the like. The computerreadable storage media may be implemented by any of a number of knowphysical devices capable of storing information representinginstructions executable via a computer such as controller 22. Knowndevices may include, but are not limited to, PROM, EPROM, EEPROM, flashmemory, and the like, in addition to magnetic, optical, and combinationmedia capable of temporary or permanent data storage.

[0032] Computer readable storage media 28 implement control logic viasoftware, firmware, hardware, micro code, and/or discrete or integratedcircuitry to affect control of various systems and subsystems of thevehicle, including the engine 12, a vehicle transmission (not shown),the turbocharger 50, and the EGR valve, and other sensors and componentsas hereinafter described. Controller 22 receives signals from sensors 20via input ports 24 and generates output signals which may be provided tovarious actuators and/or components via output ports 38. Signals mayalso be provided to a display device 40 which includes variousindicators such as lights 42 to communicate information relative tosystem operation to the operator of the vehicle.

[0033] A data, diagnostics, and programming interface 44 may also beselectively connected to the electronic engine controller module (ECM)22 via a plug 46 to exchange various information therebetween. Interface44 may be used to change values within the computer readable storagemedia 28, such as configuration settings, calibration variables, faultthreshold values, action threshold values, control logic, look-uptables, calibrated constants, and the like.

[0034] In operation, controller 22 receives signals from sensors 20 andexecutes control logic to control one or more variable geometryturbochargers by controlling an actuator capable of changing the currentturbocharger geometry so as to track a desired turbocharger geometry.The desired turbocharger geometry is determined based on any number ofengine conditions and/or parameters indicative of engine conditions. Forexample, an engine speed parameter indicative of engine speed, afiltered rate of change of the engine speed parameter, an engine torqueparameter indicative of current engine torque demand, and/or a rate ofchange of the engine torque parameter may be used as a basis for thedesired turbocharger geometry. Other engine conditions and/or parametersindicative of such conditions may be used as desired. And, as furtherdescribed herein, the recirculated exhaust gas flow rate and/orrecirculated exhaust gas ratio determined by the system and method ofthe present invention may also be used as an input to the controller tobe utilized as a basis for determined desired turbocharger geometry.

[0035] As thus far described, the system of the present invention isknown and is commercially available. In one embodiment, the ECM 22 is aDDEC controller available from Detroit Diesel Corporation, Detroit,Mich. Various other features of this controller are described in detailin U.S. Pat. Nos. 6,000,221, 5,477,827, and 5,445,128, the disclosuresof which are hereby incorporated by reference in their entirety.

[0036] With continuing reference to FIG. 1, the system 10 also includesan exhaust gas recirculation (EGR) valve 60 mounted in the exhaust line56 and operable to divert a selectable portion of the exhaust gasthrough line 62 for recirculation and combination with the charged airsupplied by the VGT 50 through intake line 58. The system furtherincludes an intake manifold pressure sensor (shown as 54 in FIG. 2) asone of the sensors. And, as will be described in further detailhereinafter, a plurality of temperature sensors 64, 66, and 68, and apressure sensor 70 are mounted at selected points in lines 56, 58, and62 to provide temperature and pressure information which is utilized bythe logic of the present invention to measure recirculated exhaust gasflow in the system 10. It should be noted that the pressure andtemperature sensors employed in the system of the present invention maybe any of a variety of commercially available sensors, selected to suitthe particular engine operating conditions for the engine with which thesystem is implemented.

[0037]FIG. 2 schematically illustrates the recirculated exhaust gas flowsystem of the present invention in greater detail. EGR 60 is controlledvia an actuator 82. In one embodiment, this actuator is a pneumaticactuator which is activated by a solenoid valve 84, connected to outputsfrom the ECM 22 to receive suitable control signals to regulate pressurefrom a compressed air supply 86 to pneumatically activate and deactivatethe EGR valve actuator 82 to position the EGR valve 60 as desired. Inone embodiment, the EGR valve 60 is controlled to move between a closedposition (i.e., none of the exhaust gas is diverted for recirculationinto the charge air), and a single, factory-selected open position whichdiverts a portion of the gas for recirculation. Alternatively, the EGRvalve 60 may be provided with a plurality of discrete controllable vanepositions, or an infinitely positionable vane which may be controlled asdescribed herein to vary the mix of recirculated exhaust gas and chargedair. The remainder of the exhaust gas is supplied via line 56 to drivethe turbine component 88 of the VGT, thereby powering the compressorcomponent 90 of the VGT to supply compressed air to the engine throughintake line 58. The VGT is typically also controlled by an actuator,such as pneumatic actuator 92 which, in one embodiment is activated by aPVH valve 94 controlled by input signals from the ECM 22. A turbochargerspeed sensor 96 may be connected to the VGT to provide VGT speedinformation to the ECM 22.

[0038] With continuing reference to FIG. 2, one embodiment of thepresent invention employs an obstruction, such as obstructor 80 in therecirculated exhaust gas line 62. In this embodiment, the pressuresensor 70 is a commercially available differential pressure sensor,which includes two pressure measurement taps (shown as P₁ and P₂ in FIG.3) mounted to sense the pressure downstream and upstream, respectively,of the obstructor 80. A temperature sensor 68 is mounted in recirculatedexhaust gas line 62 to provide exhaust gas temperature data to the ECM22. The control logic of this embodiment of the present invention(illustrated in FIG. 5) utilizes the differential pressure data providedby the differential pressure sensor 70, the exhaust gas temperature dataprovided by temperature sensor 68, and intake manifold pressure dataprovided by the pressure sensor 54 to determine EGR flow.

[0039] Referring now to FIG. 5, the EGR flow is determined by thecontrol logic utilizing the voltage input from the differential pressuresensor as follows: in one embodiment the pressure is calibrated linearlybetween 0.5 and 4.5 volts for pressure ranging from 0 to 5 pounds persquare inch. The correlation, in kPa to voltage is shown as thefollowing equation:

Differential Pressure Drop, kPa=a * (Sensor Voltage)−b  (1)

[0040] where a and b are constants which are calibrated by flow benchtrials.

[0041] The differential pressure drop is related to EGR flow ratethrough the following correlation:

EGR Flow Rate (kg/min)−(Density/Density Correction)^(c) * d *(Differential Pressure Drop, kpa)^(e)   (2)

[0042] where the Density Correction, c, d, and e are also constantswhich may be calibrated for a particular obstructor geometry by flowbench trials.

[0043] The EGR gas density is calculated from the EGR temperature,provided by sensor 68 and intake manifold pressure, provided by sensor54, according to the following equation:

Density=(Intake Pressure,kPa)/(EGR Temperature, K * 0.2876)  (3)

[0044] One embodiment of the obstructor 80, illustrated in greaterdetail in FIGS. 3 and 4, is a thin walled plate 100 which defines acircular orifice 102. The plate 100 is mounted within line 62 toobstruct EGR flow and, thereby, create a pressure differential upstreamand downstream of the plate 100. It is, of course, desirable that thepressure differential information provided by sensor 70 be accuratethroughout the lifetime of the system. It is, therefore, desirable toemploy an obstructor which inhibits the deposition of dieselparticulates in the recirculated exhaust gas stream upon the obstructor,since these deposits, over time, could effectively change the size ofthe orifice 102 and, thereby, affect the accuracy of the EGR flowdetermination. The thin wall plate 100 illustrated in FIGS. 3 and 4 isalso preferably provided with a sharp edge to minimize particulatedeposits.

[0045] As illustrated in FIG. 6, though the ideal geometry of theorifice edge is shown at 106, the edge of the plate is preferablydesigned to have a flat portion 108 and a bevel 110. This designprovides a relatively sharp edge that insures that the plate can bereliably manufactured to provide the desired orifice diameter. Incontrast, fabrication of the sharper edge 106 is likely to yield agreater variability in orifice size when machining the edge to a point.

[0046] It will be appreciated that it is desirable to design the orifice102 in such a manner so as to minimize the pressure drop while stillyielding reliable pressure differential data for the EGR flowdetermination. It has been found that the orifice is effective when itsdiameter ranges from about 40% to about 80% of the pipe diameter 106.Preferably, the orifice size is about 60% of the pipe diameter 106.

[0047] The exit edge of the orifice 114 is Preferably provided with abevel having an angle that minimizes particulate deposits on the orificeedge. It has been found that a bevel angle, α, of from about 30° toabout 60°, and preferably about 45°, effectively inhibits particulatedeposits on the orifice edge. In one embodiment, the thickness, τ, ofthe orifice plate is approximately 5% of the pipe diameter 106.

[0048] It should be appreciated that the present invention may utilizedifferential pressure information sensed upstream and downstream of anyobstruction in the recirculated exhaust gas line 62 without departingfrom the spirit of the present invention. For example, instead of usingan obstructor 80 of a particular design, the pressure sensors may bemounted as shown at 18 to determine the pressure drop across the exhaustgas cooler 120, across the EGR valve 60 (as shown at 124), or betweenthe intake and exhaust line of one of the engine cylinders (at 122), orat any other inherent obstacle in the recirculated exhaust gas path. Ofcourse, the calibrated constants will be different for each type ofobstruction, since they depend upon the geometry of the obstruction. Forexample, the pressure drop across the EGR valve 60 is calculatedaccording to the following equation:

EGR Flow Rate (kg/min)=(Cd Function) * (Density/DensityCorrection)^(f) * G * (Differential Pressure Drop, kpa)^(h)   (4)

[0049] where f, G, and h are calibratable constants, and where CdFunction is the discharge coefficient of the EGR valve.

[0050] It will be appreciated that, although the calibration of the EGRvalve may be more complex than the thin plate obstructor embodiment,using an inherent obstruction eliminates the need to add a designedobstructor, thereby reducing the EGR loop pressure drop and the fuelpenalty associated with such a pressure drop.

[0051] Referring now to FIG. 7, the ratio between the mass flow ofrecirculated exhaust gas and charge air may be calculated as a functionof the charge air temperature, recirculated exhaust gas temperature, andcharge air/exhaust gas mixture temperature. Thus, temperature sensors 66and 64 may be mounted to sense charge air temperature and charge airexhaust gas mixture temperature, respectively, and utilized in additionwith temperature data sensed by temperature sensor 64 (as shown in FIG.2) to determine an EGR rate: $\begin{matrix}{{{EGR}\quad \%} = {\frac{{\overset{.}{m}}_{egr}}{{\overset{.}{m}}_{air}} = \frac{h_{mixture} - h_{air}}{h_{egr} - h_{mixture}}}} & (5)\end{matrix}$

[0052] the EGR rate calculation can be further simplified with theapproximation that entropy, h, equals temperature (in degrees Kelvin).With this assumption, the equation becomes: $\begin{matrix}{{{EGR}\quad \%} = {\frac{{\overset{.}{m}}_{egr}}{{\overset{.}{m}}_{air}} - \frac{T_{mixture} - T_{air}}{T_{egr} - T_{mixture}}}} & (6)\end{matrix}$

[0053]FIG. 8 illustrates the logic employing this temperaturedifferential method of determining the EGR ratio.

[0054] It should be appreciated that the temperature differential methodhas the advantage of being less intrusive than the differential pressuremethod, since there is no pressure drop associated with an obstructor.However, current temperature sensors are not as quickly responsive tochanging transient conditions and, therefore, the system employing thetemperature differential method is currently less desirable formeasuring exhaust gas flow in an engine which has transient operatingconditions in its normal use. However, in one embodiment of the presentinvention both the temperature differential and pressure differentialsystem may be simultaneously employed to separately and independentlydetermine recirculated exhaust gas flow, thereby providing the EGM withthe capability of cross calibrating the differential pressuremeasurement with the differential temperature measurement, particularlyduring steady state operating conditions.

[0055] The present invention thus provides a simple, reliable system andmethod for measuring recirculated exhaust gas flow in a turbochargedcompression ignition engine. This measurement may be used by the ECM tooptimize the mixture of recirculated exhaust gas with charge air tooptimize emissions and engine performance objectives. In particular, thesystem of the present invention may be utilized to provide recirculatedexhaust gas flow measurement data to EGR valve control logic and/or VGTcontrol logic to provide closed loop feedback control of these systemcomponents. One embodiment of an engine control system which may employthe system and method of the present invention for closed-loop feedbackcontrol of the EGR valve and/or turbocharger is disclosed in patentapplication Ser. No. 09/540,017, for a “METHOD OF CONTROLLING AN ENGINEWITH AN EGR SYSTEM,” filed concurrently herewith, naming S. MillerWeisman, II, Admir Kreso, and Andrew May as inventors, the disclosure ofwhich is incorporated by reference herein in its entirety.

[0056] While embodiments of the invention have been illustrated anddescribed, it is not intended that these embodiments illustrate anddescribe all possible forms of the invention. Rather, the words used inthe specification are words of description rather than limitation, andit is understood that various changes may be made without departing fromthe spirit and scope of the invention.

What is claimed is:
 1. A method for measuring the mass flow rate ofrecirculated exhaust gas in a vehicle having a compression-ignitionengine, a plurality of engine sensors having outputs indicative ofcurrent engine conditions including an intake manifold pressure sensor,and a variable geometry turbocharger in which geometry is varied by acontrollable actuator, the method comprising: sensing intake manifoldpressure; controllably diverting a selected portion of exhaust gas formixture with intake air; sensing temperature in a flow of therecirculated exhaust gas; obstructing the flow of the recirculatedexhaust gas; sensing pressure of the recirculated exhaust gas downstreamof the obstruction; sensing pressure of the recirculated exhaust gasupstream of the obstruction; and determining flow rate of therecirculated exhaust gas as a function of the intake manifold pressure,recirculated exhaust gas temperature, and differential pressure dropacross the obstruction.
 2. The method of claim 1 wherein the exhaust gasis recirculated through a pipe, and wherein the flow of the recirculatedexhaust gas is obstructed by providing a thin plate obstructor mountedin the pipe in the path of the recirculated exhaust gas defining anorifice having a diameter of between about 40% and about 80% of thediameter of the recirculated exhaust gas pipe.
 3. The method of claim 2further including providing a sharp edge on the edge of the plate whichdefines the orifice.
 4. The method of claim 3 further includingbevelling the edge of the obstructor defining the orifice on thedownstream side of the orifice at an angle of between about 30° andabout 60° from horizontal.
 5. The method of claim 4 including bevellingthe edge of the obstructor defining the orifice on the downstream sideof the orifice at an angle of about 45°.
 6. The method of claim 2wherein the flow of the recirculated exhaust gas is obstructed byproviding a thin plate obstructor mounted in the pipe in the path of therecirculated exhaust gas defining an orifice having a diameter of about60% of the diameter of the recirculated exhaust gas pipe.
 7. The methodof claim 1 wherein the flow rate of the recirculated exhaust gas isdetermined by determining the differential pressure drop across theobstruction according to the following relationship: DifferentialPressure Drop, kPa=a * (Sensor Voltage)−b; determining recirculatedexhaust gas density according to the following relationship:Density=(Intake Pressure,kpa)/(EGR Temperature, K * 0.2876); anddetermining recirculated exhaust flow rate according to the followingrelationship: EGR Flow Rate (kg/min)−(Density/Density Correction)^(c) *d * (Differential Pressure Drop, kPa)^(e), where a, b, c, d, and e arecalibratable constants.
 8. A method for measuring flow ratio ofrecirculated exhaust gas to charge air in a vehicle having acompression-ignition engine, a plurality of engine sensors havingoutputs indicative of current engine conditions, and a variable geometryturbocharger in which geometry is varied by a controllable actuator, themethod comprising: diverting a selected portion of exhaust gas formixture with intake air; sensing the temperature of the recirculatedexhaust gas; sensing the temperature of the charge air; sensing thetemperature of the mixture of charged air and recirculated exhaust gas;and determining the flow ratio of the recirculated exhaust gas as afunction of the temperature of the charge air, the temperature of therecirculated exhaust gas, and the temperature of the mixture of chargeair and recirculated exhaust gas.
 9. The method for claim 8 whereindetermining the flow ratio of the recirculated exhaust gas is based uponthe relationship: $\begin{matrix}{{{EGR}\quad \%} = {\frac{{\overset{.}{m}}_{egr}}{{\overset{.}{m}}_{air}} - {\frac{T_{mixture} - T_{air}}{T_{egr} - T_{mixture}}.}}} & (6)\end{matrix}$


10. An information recording medium for use in a control module thatcontrols a compression-ignition engine including a plurality of enginesensors having outputs indicative of current engine conditions, anintake manifold pressure sensor, and a variable geometry turbocharger inwhich geometry is varied by a controllable actuator, the informationrecording medium recording a computer program that is readable andexecutable by the control module, the computer program comprising:sensing the intake manifold pressure; controllably diverting a selectedportion of the exhaust gas from the engine exhaust line for mixture withintake air; sensing the temperature in the flow path of the recirculatedexhaust gas; sensing the pressure of the recirculated exhaust gasdownstream of an obstruction in the flow of the recirculated exhaustgas; sensing the pressure of the recirculated exhaust gas upstream ofthe obstruction; and determining flow rate of the recirculated exhaustgas as a function of the current intake manifold pressure, recirculatedexhaust gas temperature, and differential pressure drop across theobstruction.
 11. The information recording medium of claim 10 whereinthe computer program further comprises determining the flow rate of therecirculated exhaust gas by determining the differential pressure dropacross the obstruction according to the following relationship:Differential Pressure Drop, kPa=a * (Sensor Voltage)−b; determining therecirculated exhaust gas density according to the followingrelationship: Density=(Intake Pressure,kpa)/(EGR Temperature, K *0.2876); and determining the recirculated exhaust flow rate according tothe following relationship: EGR Flow Rate (kg/min)−(Density/DensityCorrection)^(c) * d * (Differential Pressure Drop, kPa)^(e), where a, b,c, d, and e are calibratable constants.
 12. An information recordingmedium for use in a control module that controls a compression-ignitionengine including a plurality of engine sensors having outputs indicativeof current engine conditions, an intake manifold pressure sensor, and avariable geometry turbocharger in which geometry is varied by acontrollable actuator, the information recording medium recording acomputer program that is readable and executable by the control module,the computer program comprising: controllably diverting a selectedportion of the exhaust gas for mixture with intake air; sensing thetemperature of the recirculated exhaust gas; sensing the temperature ofthe charge air; sensing the temperature of the mixture of charged airand recirculated exhaust gas; and determining the flow ratio of therecirculated exhaust gas as a function of the temperature of the chargeair, the temperature of the recirculated exhaust gas, and thetemperature of the mixture of charge air and recirculated exhaust gas.13. The information recording medium of claim 12 wherein the computerprogram further comprises determining the flow ratio of the recirculatedexhaust gas based upon the relationship: $\begin{matrix}{{{EGR}\quad \%} = {\frac{{\overset{.}{m}}_{egr}}{{\overset{.}{m}}_{air}} - \frac{T_{mixture} - T_{air}}{T_{egr} - T_{mixture}}}} & (6)\end{matrix}$